A wireless communication device such as a mobile phone which is provided with a voltage controlled oscillation circuit is widely used. The oscillation frequency of the voltage controlled oscillation circuit may be varied by a control voltage.
In a conventional voltage controlled oscillation circuit, there is a problem that phase noise performance deteriorates by a noise when the amplitude of the oscillation output signal is too small. There is also a problem that the phase noise performance deteriorates by a distortion of the oscillation output signal when the amplitude of the oscillation output signal is too large.
In the case that the voltage controlled oscillation circuit is provided with a metal-oxide semiconductor field effect transistor (hereinafter, referred to as “MOSFET”), the phase noise performance deteriorates because distortion occurs in the oscillation output signal, when the MOSFET operates within a linear operation area, i.e. when the amplitude of the oscillation output signal is larger than a threshold voltage of the MOSFET.
The amplitude of the oscillation output signal may be adjusted by regulating a bias current of the voltage control oscillation circuit so as to be substantially equal to the threshold voltage of the MOSFET.
However, it causes a problem that the phase noise performance fluctuates because the amplitude of the oscillation output signal fluctuates by the changes of an operation condition such as bias current or threshold voltage of the MOSFET.
The changes of the operation condition occur by the influence of use environment such as operation temperature or power supply voltage, and of oscillation frequency.
Even if the amplitude of the oscillation output signal is fixed to a value, there is still a problem that the phase noise performance fluctuates because the threshold voltage of the MOSFET changes in the fabrication process.
A voltage controlled oscillation circuit which suppresses the phase noise is proposed in U.S. Pat. No. 5,834,983.
This voltage controlled oscillation circuit fixes the amplitude of the oscillation output signal to a reference signal which shows a maximum level immediately before the distortion begins to occur in the oscillation output signal.
This voltage controlled oscillation circuit comprises a constant collector bias current source, a variable collector bias voltage circuit and a voltage tunable resonant circuit.
The variable collector bias voltage circuit detects the collector output amplitude of a transistor of the variable collector bias voltage circuit. The variable collector bias voltage circuit regulates the collector bias voltage using the integration value of the difference between the collector bias voltage and the reference signal. By the regulation, the variable collector bias voltage circuit controls the amplitude of the oscillation output signal so as to become constant.
The operation point of the transistor is kept at a constant relation to an emitter cut-off, because a collector bias current is kept constant by the constant collector bias current source.
The collector voltage is high sufficiently in accordance with the initial setting values of the constant collector bias current corresponding to each resonated voltage, and with the reference signal. Consequently, the collector bias current is controlled to a state that the moment collector current is substantially zero, i.e. to a state immediately before emitter cut-off.
The voltage controlled oscillation circuit described above causes a problem that a sufficient phase noise performance is not obtained depending on the variation in the transistor fabrication process.
It is because the amplitude of the oscillation output signal is controlled to keep a prediction value. The prediction value is not the amplitude of the oscillation output signal but a bias voltage, which is predicted to minimize the phase noise by detecting a maximum or a minimum value of the oscillation output signal.
Another voltage controlled oscillation circuit is proposed in U.S. Pat. No. 6,653,908. In the voltage controlled oscillation circuit, the amplitude of the oscillation output signal is automatically controlled so that the phase noise may be minimized.
This voltage controlled oscillation circuit comprises an oscillation circuit having a bipolar transistor, a negative peak detector to detect a minimum value of the collector voltage of the transistor, and a positive peak detector to detect a maximum value of the emitter voltage of the transistor.
The bias current of the voltage controlled oscillation circuit is controlled by a feedback of the bias current so that the difference between the minimum value of the collector voltage and the maximum value of the emitter voltage is substantially equal to a saturation voltage of the bipolar transistor.
In the voltage controlled oscillation circuit of U.S. Pat. No. 6,653,908 described above, it is indefinite how saturation voltage is produced. Further, any circuit structure, which utilizes a MOSFET for the transistor is not disclosed.